Our current mechanistic understanding of sensory representation in primary visual cortex (V1) is principally ?feed-forward?. That is, the spatio-temporal structure of the receptive field (RF) of V1 neurons results from the combination of receptive filed elements transmitted by earlier stages of visual processing (i.e. retina and thalamus), in a feed-forward manner. While computational models based on this feed-forward understanding of V1 are relatively accurate at predicting the response of neurons to visual stimuli presented within the borders of the neuron?s RF, they generally fail when the stimuli exceed the size of the RF, as is the case for every-day visual scenes that encompass the entire field of view. One of the main reason for the inability of current models to predict responses of V1 neuron to naturalistic stimuli is that the response of neurons in V1 to stimuli in their RF is strongly modulated by what happens outside of their RF. In other words, how neurons in V1 respond to a stimulus presented in their RF depends on the context or ?surround? within which the stimulus is presented. In the natural world, visual stimuli falling in the RF of a V1 neuron are never devoid of a surround. Consistent with these neurophysiological observations, psychophysical experiments in humans and animals show that the perception of a visual stimulus depends on its relationship to the surrounding visual context. Thus, the ability to process visual information depending on the context is a key property of visual cortex and has a profound impact on how we perceive the world. Only a clear mechanistic, circuit level understanding of how the context modulates the response of V1 will allow us generate realistic models capable of accurately predicting the response of this area to naturalistic stimuli. Neurophysiological data indicate that contextual modulation of neuronal responses in V1 most likely relies on intra-cortical interactions. That is, while the classical RF structure of a V1 neuron results from a feed-forward process, the modulation by the surround likely depends on intra- cortical circuit elements, their connectivity pattern and their dynamic interactions. This proposal aims at elucidating these cortical circuit elements and how their interaction gives rise to the contextual modulation of the response to visual stimuli in V1. A thorough mechanistic understanding of contextual modulation in V1 will allow us to build realistic models capable of capturing the response of visual cortex to naturalistic stimuli. Thus, data obtained from this proposal will be essential in informing and validating realistic models of V1.